The present invention relates to the field of electrical test probes. More particularly, the present invention relates to an active differential test probe with a transmission line input structure.
An electrical signal will change when a test instrument, such as an oscilloscope, is connected to the circuit that generates the signal. For example, if a bare wire is connected between a circuit and an oscilloscope, the wire and the input circuitry of the oscilloscope effectively add a load resistance and a shunt capacitance to the circuit. This reduces the measured voltage and affects measurements of dynamic timing characteristics, such as pulse rise time. For this reason, a test probe that minimizes the loading effects on the circuit is generally used when a test instrument is connected to a circuit. Several general types of test probes have been developed.
With a high-impedance test probe, it is possible to take a small sample of the signal without appreciably loading the circuit being measured. A high-impedance test probe consists of a large value resistor and an input capacitor coupled, in parallel, to a test point in the circuit. A high-impedance test probe, however, is not suited for high-frequency measurements because of the relatively high value of its input capacitance.
A low-impedance test probe is better suited for measurement of high frequency signals. A low-impedance test probe consists of a low-value input resistor in series with the signal conductor of a low-loss coaxial cable that is treated as a terminated transmission line. One limitation of the low-impedance test probe is that it may be used only at a test point with a relatively low source resistance. Another disadvantage is that the low-impedance test probe is a single ended test probe. An additional limitation of the low-impedance test probe is that the frequency is limited to the resonant frequency of the probe input capacitance in series with the ground lead and probe tip inductance.
Probing of high speed electrical signals has long been a challenge for designers of electronic test equipment. As signal speeds increase, test equipment must similarly increase in speed to keep up. The interface between the electrical signal and the test equipment is the probe. Ideally, the probe would transmit the received electrical, without acting upon it at all, to the associated electronic test equipment. However, because the probe has physical properties associated with it, this ideal situation cannot physically be achieved.
In particular, a number of different elements within a probe generate various capacitances and inductances that act upon the electronic signal being transmitted. As the electronic signal speed increases, the effects of these elements are more pronounced.
Therefore, it would be beneficial to provide a probing solution that allowed for probing of high speed signals while reducing the effect of the probe on the electronic signal being transmitted.